An overwhelming number of new drugs, surgical procedures, behavioral psychiatric and physical techniques is now available for the management of pain. There is little evidence that any of these interventions are of value except for highly select homogeneous groups of patients. Evaluation of these interventions by controlled experimentation is most difficult due to ethical considerations and the large number of variables involved. The most promising approach for evaluating effectiveness appears to be one derived from the accumulation of data on the outcome of treatments applied to actual pain patients. The potential usefulness of this type of deta is limited by the lack of an adequate method for specifying experienced clinical pain, and, especially, by the lack of an appropriate methodology for objectively classifying patients into homogeneous groups. The proposed program of research has the following long term goals: (1) to develop a representation system that specifies the perceived relationships between the words people use to describe their pains to others (2) to develop methods for specifying both the qualitative and quantitative characteristics of experienced pain as well as other characteristics associated with the experience of pain (3) to develop a method for specifying intensity or severity that controls for patients' variability in verbal response tendencies when expressing a complaint of pain (4) to develop procedures for identifying subgroups of patients and subsets of variables such that the patients within a subgroup are homogeneous with respect to variables within a subset (5) to apply the results obtained in the first four goals to the evaluation of different types of treatments for chronic, low-back pain patients.